Install 2 filters. The first one from the vacuum shutoff to the ignition key and the second from the vacuum pump to brake booster line, on one of the 2 lines that comes off it. These 2 positions will tell you if the shutoff vallve or the vacuum pump is leaking before it gets into the components and ruins them. Here is the 240D with them installed. I used a lawnmower filter. good size and small.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car